One of the most important parameters needed to define the origin of life and track it from the oldest record of the earth to present is the ability to define the rise of oxygen and quantify its evolution. A mechanism to provide this information had been sought for decades but was notoriously difficult to define. The discovery of mass independent isotopic anomalies of the four stable isotopes of sulfur in the earths earliest rock record provided definition of the origin of life from the emergence of oxygen. It has been shown by experiments and quantum chemical theory that this arises from tropospheric UV photolysis of sulfur dioxide. Tropospheric UV photolysis in todays tropopshere does not occur due to UV filtration by stratospheric ozone. The occurence of the mass independent UV isotopic anomlay in the early earth occurs due to significantly lower oxygen and ozone levels and penetration of UV light to the tropopshere to photolyze SO2, consequently providing a measure of atmopsheric oxygen levels. Providing deeper understanding of the photochemistry of sulfur dioxide at the most basic quantum chemical level alllows more prescise definition of the Archean sulfur isotopic anomalies and calculation of atmopsheric oxygen levels and associated chemistry. Recently, we added measurement of a fifth sulfur isotope, radiogenic 35S, a cosmic ray produced radionuclide with a 87 day half life. From measurements of all five isotopes in the present day atmopshere we have been able to provide significantly deeper chemical mechanistic information during the Pre Cambrium as well as the present atmopshere. The radiogenic and stable isotope uniquely defines the stratopsheric UV contribution and a new significant tropospheric component.These measurements, plus measurements of polar ice sulfates and volcanic upper atmospheric processes provides a combined powerful approach to interpreting the ancient atmosphere, oxygen origins and levels as well as definition of sulfur chemical processes in todays atmopshere.

Five-isotope approach to tracking the origin and evolution of life

2019 Fall National Meeting

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